Materials which contract on heating (negative thermal expansion materials)
are of great importance since they can be mixed with other materials (positive
thermal expansion materials) to make composites which have zero thermal
expansion. Materials with zero or low thermal expansion have many applications
in electronic devices, space telescope mirrors, stove tops, cook wares and
so on. The purpose of this research was to understand why some network
structures show anomalous (very low or negative) thermal expansion. Isotropic
materials have isotropic thermal expansion which means that on heating,
the structure will contract (or expand) the same in all dimensions. Examples
of isotropic materials with low or negative thermal expansion are ZrP2O7 and ZrV2O7 respectively. We have solved the structures of ZrP2O7 and ZrV2O7, and found that ZrP2O7
shows normal thermal expansion up to about 290 °C at which the phase
transition occurs,and very low and positive thermal expansion after 290
°C. However, ZrV2O7 shows
positive thermal expansion up to about 100 °C, and very strong negative
thermal expansion after 100 °C up to about 800 °C at which the material
decomposes. There are two phase transitions observed for ZrV2O7 before 100 °C which makes the structure more challenging.
The structure of both ZrP2O7
and ZrV2O7 is cubic and the
space group is Pa3. A 3 x 3 x 3 superstructure is observed for both
compounds at room temperature. The 3 x 3 x 3 superstructure disappears
after the phase transition is achieved in ZrP2O7, but it doesn't fully disappear until the second phase
transition is achieved in ZrV2O7.
We believe that the unusual thermal expansion of these compounds is a result
of frustration in bending some of the P-O-P or V-O-V angles away from 180°.
Reference : Khosrovani, N.; Sleight A.W., Inorg. Chem. 35,
485 (1996)